1. Field of the Invention
The present invention relates to a an integrated circuit (IC) light-receiving element in which a circuit for processing a photoelectrically converted signal is integrated. More particularly, the present invention relates to a structure for improving the response speed of a light-receiving element such as a divided photodiode element usable for an optical pickup and the like.
2. Description of the Related Art
An optical pickup has heretofore been used for various kinds of optical disk devices including a CD-ROM, a digital video disk (DVD) and the like. In recent years, CD-ROM devices have been developed more and more actively and rapidly so as to have an even higher response speed. Currently, CD-ROM devices having a 4.times. to 6.times. response speed (i.e, a response speed 4 to 6 times as high as a normal response speed) have been put on the market. Furthermore, CD-ROM devices having a 8.times. to 12.times. response speed have been developed to be commercialized in the near future. DVD have also been developed remarkably. In a DVD, it is possible to gain access to data stored therein at a response speed approximately as high as 6.times. response speed of a CD-ROM. It is very probable that a DVD having a response speed twice as high as that of a currently available DVD would be developed in the near future.
Furthermore, such optical disk devices are now required to process a large amount of data necessary for storing a motion picture and the like. In view of these circumstances, it is an urgent task to realize a high response speed for an optical pickup.
A divided photodiode element, in which a light-receiving region is divided into a plurality of light detection areas, has conventionally been used as a signal detector element for an optical pickup.
As a high-performance optical disk device of a smaller size has been realized in recent years, it has become more and more important to reduce the size and the weight of an optical pickup. In order to realize such an optical pickup, an optical module, in which a tracking beam generation function, a light branching function and an error signal generation function are integrated into one hologram element and a laser diode and a divided photodiode element are provided within a package, has been proposed.
FIG. 7 schematically shows an exemplary arrangement for such an optical system 1000 for optical pickup. Hereinafter, it will be briefly described how the optical system 1000 detects a signal in principle.
Light emitted from a laser diode 110 is incident onto a hologram element (the external configuration of the element is not shown in FIG. 7) having a diffraction grating 120 and a hologram 130 which are formed on the lower surface and the upper surface thereof, respectively. The light emitted from the laser diode 110 is separated by the diffraction grating 120 formed on the lower surface of the hologram element for generating a tracking beam. The tracking beam is split into three optical beams consisting of two sub-beams used for tracking and one main beam for reading out an information signal. Then, the light beams are transmitted through the hologram 130 formed on the upper surface of the hologram element as zero-order light, transformed by a collimator lens 140 into parallel light beams and then condensed by an objective lens 150 onto a disk 160.
The condensed light is modulated and reflected by the pits formed on the disk 160; transmitted through the objective lens 150 and the collimator lens 140; and then diffracted by the hologram 130 so as to be guided as first-order diffracted light beams onto a divided photodiode element 170, on which five divided light-detecting photodiode sections D1 to D5 (hereinafter, simply referred to as "light-detecting sections") are formed. In this case, the divided photodiode element 170 functions as a light-receiving element in the optical system 1000 for optical pickup.
The hologram 130 includes two regions having respectively different diffraction periods. When the reflected light beam of the main beam is incident onto one of the two regions, the light beam is condensed onto a linear isolating section between the light-detecting sections D2 and D3. On the other hand, when the reflected light beam of the main beam is incident onto the other region of the hologram 130, the light beam is condensed onto the light-detecting section D4. The reflected light beams of the two sub-beams are condensed onto the light-detecting sections D1 and D5, respectively.
In the optical system 1000, the incidence positions of the main beam on the divided photodiode element 170 is moved along the direction perpendicular to the longitudinal direction of the light-detecting sections D2 and D3 in accordance with the variation of the distance between the hologram 130 and the disk 160. In the case where the ma in beam is in focus on the disk 160, the reflected light beam thereof is incident onto the isolating section between the light-detecting sections D2 and D3. Thus, assuming that the outputs of the light-detecting sections D1 to D5 of the divided photodiode element 170 are denoted by S1 to S5, respectively, a focusing error signal FES is given based on the equation: FES=S2-S3.
On the other hand, a tracking error is detected by a so-called three-beam method. Since the two, sub-beams for tracking are condensed onto the light-detecting sections D1 and D5, respectively, a tracking error signal TES is given based on the equation: TES =S1-S5. Thus, when the tracking error signal TES is zero, the main beam is correctly located on the target track to be irradiated by the main beam.
Furthermore, a reproduced signal RF is given as a sum of the outputs of the light-detecting sections D2 to D4 for receiving the reflected light beam of the main beam based on the equation: RF=S2+S3+S4.
FIG. 8 shows a plan view of the divided photo-diode element 170 of the optical system 1000.
In the divided photodiode element 170, five light-detecting sections D1 to D5 are formed within an elongated region, as described above. In addition, as shown in FIG. 8, a pair of anode electrodes 172a and 172b which are commonly used for all of the light-detecting sections D1 to D5 and five cathode electrodes 174a to 174e corresponding to the five light-detecting sections D1 to D5, respectively, are disposed so as to surround the region in which the light-detecting sections D1 to D5 are provided.
The shape of the divided photodiode element 170 is determined by the optical system 1000. As shown in FIG. 8, all of the light-detecting sections D1 to D5 have an elongated shape. The reason is as follows.
In assembling the optical system 1000, after the laser diode 110 and the divided photodiode element 170 are incorporated into one package, the hologram element having the hologram 130 and the diffraction grating 120 is adhered to the upper surface of the package. During this assembly, an error is likely to be caused in aligning the positions of the laser diode 110 and the divided photodiode element 170. In addition, the oscillation wavelength of the laser diode 110 is variable not only among the individual products thereof but also in accordance with the variation in the temperature. Because of these reasons, the diffraction angle of the diffracted light is varied, so that the incidence position of the diffracted light deviates in some cases. In order to deal with such problems, the light-receiving planes of the divided photodiode element 170 are required to have a longer side along the Y direction shown in FIG. 8, i.e., a direction along which the incidence position of the diffracted light is moved in accordance with the variation in the diffraction angle.
On the other hand, in the X direction shown in FIG. 8, the diffraction angle of the diffracted light is not affected by the variation in the oscillation wavelength which results from the variation in the oscillation wavelength of the laser diode 110 among the individual products thereof or the variation in the oscillation wavelength caused by the variation in the temperature. In addition, an error caused when the positions of the laser diode 110 and the divided photodiode element 170 are aligned can be compensated for by rotating the hologram element to be adhered to the upper surface of the package. Thus, the light-receiving planes of the divided photodiode element 170 are not required to have a longer side along the X direction. To the contrary, if the distance between adjacent ones of the three light beams to be incident in parallel to each other along the X direction is large, then it is difficult to adjust the position of the optical pickup to be incorporated into the optical disk device. Thus, regarding the X direction, the widths of the light-detecting sections D1 to D5 and the widths of the isolating sections among the light-detecting sections D1 to D5 are required to be small.
In view of the above-described respects, the shape of the divided photodiode element 170 necessarily becomes elongated.
FIG. 9 shows a schematic cross-sectional view of a conventional light-detecting divided photodiode element 170 as seen along the line IX--IX shown in FIG. 8. It is noted that various components including a multi-layer wiring, a protective film and the like to be formed during the respective steps succeeding a metal wiring processing step are omitted in FIG. 9.
Hereinafter, a method for fabricating the divided photodiode element 170 will be described with reference to the cross-sectional views illustrated as FIG. 10A to 10D. In FIG. 9 and FIGS. 10A to 10D, the same components are identified by the same reference numerals.
First, P-type buried diffusion regions 2 are formed in the regions in the vicinity of the surface of a P-type semiconductor substrate 1 which are to be the isolating sections for isolating the light-detecting sections D1 to D5 from each other (FIG. 10A).
Next, as shown in FIG. 10B, an N-type epitaxially grown layer 4 (hereinafter, simply referred to as an "N-type epi-layer") is formed over the entire surface of the P-type semiconductor substrate 1. Then, P-type isolating diffusion regions 5 are formed in the respective regions inside the N-type epi-layer 4 so as to correspond to the P-type buried diffusion regions 2. The P-type isolating diffusion regions 5 are formed so as to vertically extend from the surface of the N-type epi-layer 4 to reach the upper surface of the P-type semiconductor substrate 1 (or the surfaces of the P-type buried diffusion regions 2). As a result, the N-type epi-layer 4 is divided into a plurality of electrically isolated N-type semiconductor regions, so that the respective light-detecting sections D1 to D5 are formed as shown in FIG. 10C (though the light-detecting section D4 is not shown in FIG. 10C).
Thereafter, as shown in FIG. 10C, a P-type diffusion layer 6 is formed in the surface region of the N-type epi-layer 4 between the P-type isolating diffusion region 5 on the right end and the P-type isolating diffusion region 5 on the left end so as to cover at least a portion of the upper surfaces of the P-type isolating diffusion regions 5 to be the isolating sections of the light-detecting sections D1 to D5, respectively.
Subsequently, as shown in FIG. 10D, the portion of an oxide film 7 which is formed on the surface of the P-type diffusion layer 6 and the N-type epi-layer 4 when the P-type diffusion layer 6 is formed and which corresponds to the light-receiving region on the surface of the P-type diffusion layer 6 is removed, and a nitride film 8 is formed instead above the semiconductor substrate 1. The film thickness of the nitride film 8 is set in accordance with the wavelength of the laser diode so as to function as an antireflection film.
Next, electrode windows are opened through the oxide film 7 and the nitride film 8. Then electrode wirings 9a are formed, and simultaneously, metal films 9 are formed on the portions on the surface of the nitride film 8 onto which the signal light is not irradiated, whereby the structure shown in FIG. 9 for the divided photodiode element 170 is obtained. A signal processor section (not shown) is formed on the semiconductor substrate 1 by performing an ordinary bipolar IC process.
In the respective light-detecting sections D1 to D5, when an inverse bias is applied thereto, a depletion layer 11 is formed in the vicinity of the surface of the substrate 1 as shown in FIG. 9.
In the divided photodiode element 170, the P-N junction in each isolating section between adjacent ones of the light-detecting sections D1 to D5 is covered with the P-type diffusion layer 6. As a result, even if the nitride film 8 is directly formed on the surface of the divided photodiode element 170, problems such as an increase in the junction leakage are not caused. Therefore, the condensed beam of the light reflected by the disk 160 (hereinafter, such reflected light will be referred to as "diffracted light" because the reflected light is also diffracted by the hologram 130) is not reflected so much by the light-receiving plane even in the isolating section between the light-detecting sections D2 and D3, onto which the condensed beam is actually incident. As a result, the sensitivity of the divided photodiode 170 can be improved.
In addition, since metal films 9 are formed in the sections onto which the light beams of the diffracted light are not incident (i.e., an isolating section between the light-detecting sections D1 and D2 and an isolating section between the light-detecting sections D3 and D5, in this case), the divided photodiode element 170 is less likely to be affected by the stray light or the like, so that an S/N ratio of the divided photodiode element 170 can be improved.
A high-speed operation is required, in particular, for the light-detecting sections D2, D3 and D4 for processing the reproduced signal RF. In the case where a light beam is irradiated onto the isolating section between these light-detecting sections D2 and D3, the cutoff frequency of the divided photodiode element 170, in particular, is decreased as compared with the case where light beams are irradiated onto the centers of the respective light-detecting sections.
The experimental results demonstrating the decrease in the cutoff frequency of the divided photodiode element 170 are shown in FIGS. 11A and 11B. FIG. 11A is a cross-sectional view showing the vicinity of the light-detecting sections D2 and D3 of the divided photodiode element 170 shown in FIG. 9. FIG. 11B is a graph representing the dependence of the cutoff frequency of the divided photodiode element 170 upon the position of the light beam. In FIG. 11B, the abscissas represent the positions of the light beam of the diffracted light in the vicinity of the light-detecting sections D2 and D3, while the ordinates represent the cutoff frequencies fc (MHz) at the respective positions. As shown in FIG. 11B, when the light beam is located in the vicinity of the isolating section between the light-detecting sections D2 and D3, the cutoff frequency fc is decreased.
In this case, the specific resistance of the P-type semiconductor substrate 1 is set to be 15 .OMEGA.cm and the cutoff frequency is measured under the conditions where the inverse bias to be applied to the photodiode element 170 is set to be 1.5 V and the load resistance is set to be 380 .OMEGA.. On the other hand, since the light used for a CD-ROM has a wavelength .lambda. of 780 nm and the light used for a DVD has a wavelength .lambda. of 635 nm, the cutoff frequencies of the photodiode element corresponding to these two wavelengths are measured as the experimental results.
As shown in FIG. 11B, since the response speed of the element corresponds to a cutoff frequency on the order of several MHz with respect to the light having a wavelength of 780 nm when light is irradiated onto the isolating section, the element is operable with an optical disk device for a 4.times. CD-ROM in view of the performance thereof. However, the element cannot be operated in association with a CD-ROM having a 6.times. or higher response speed.
On the other hand, since the response speed of the element corresponds to a cutoff frequency of 20-odd MHz with respect to the light having a wavelength of 635 nm when light is irradiated onto the isolating section, the element is operable with a DVD having a normal response speed. However, since a 2.times. DVD requires a photodiode element to have a cutoff frequency of 30 MHz or higher, a divided photodiode element having an element structure shown in FIG. 9 cannot be operated in association with a 2.times. DVD.
The state where a light beam has been irradiated onto the isolating section between the light-detecting sections D2 and D3 was analyzed by utilizing a device simulation. As a result, it was confirmed that the optical carriers made a detour around the P-type buried diffusion region 2 in the isolating section so as to reach the junction section between the N-type epi-layer 4 and the P-type semiconductor substrate 1. When the optical carriers follow such a detour, the diffusion movement distance of the optical carriers becomes longer, thereby causing the above-described decrease in the cutoff frequency.
In addition, a difference exists between the response speed corresponding to the light having a wavelength of 780 nm and the response speed corresponding to the light having a wavelength of 635 nm as shown in FIG. 11B, because the penetration length of the light (i.e., the penetration depth of the light into the substrate) is varied in accordance with the wavelength thereof. The light having a wavelength of 635 nm has more satisfactory response characteristics, because the light has a shorter penetration length and a shorter diffusion movement distance of the optical carriers.
FIG. 12 shows the results of the simulation for obtaining the current paths in the P-type buried diffusion region 2 corresponding to the isolating section between the light-detecting sections D2 and D3 and in the vicinity of the isolating section, in which the directions of the currents are indicated by arrows. The electrons functioning as optical carriers move in the opposite directions to those indicated by the arrows shown in FIG. 12.
FIG. 13 is a graph showing the potential distribution of the P-type isolating diffusion region 5 in the isolating section between adjacent light-detecting sections in the depth direction. As shown in FIG. 13, the potential distribution functions as a potential barrier with respect to the electrons or the optical carriers moving in the substrate 1 toward the surface region thereof. Therefore, as shown in FIG. 12, the optical carriers move while making a detour around the P-type buried diffusion region 2.
As described above, the specific resistance of an ordinarily used P-type semiconductor substrate 1 is about 15 .OMEGA.cm. Thus, in the case where the inverse bias to be applied to the light-detecting photodiode sections constituting the respective light-detecting sections is 1.5 V, the depth Xj (or the diffusion depth into the P-type semiconductor substrate 1) of a P-type buried diffusion region 2 is about 2.5 .mu.m, whereas the depth Xd of a depletion layer 11 is no greater than about 1.7 .mu.m, as shown in FIG. 9. As a result, the optical carriers run a distance of about 10-odd .mu.m while making a detour, as shown in FIG. 12.